The sends two powerful beams up over Manhattan every September, in the season when migrating birds cross the city at night. Radar and acoustic sensors tracked what the birds did inside them, and the operators switched the lights off from time to time. What differed between on and off?
Inside the beams, migration stopped being migration. Birds gathered at densities up to 20 times the surrounding sky, slowed down, flew in circles and called constantly, and the effect reached as high as 4 kilometers. Switching the lights off ended the disruption within minutes. Across 7 nights spread over 7 years, the installation influenced an estimated 1.1 million birds.
Billions of birds migrate at night through skies that keep getting brighter, taking their from cues that artificial light can overwhelm. What makes this installation testable is that it goes on and off, which almost no light pollution does.
and acoustic sensors recorded what happened in and around the beams. Birds aggregated in densities more than 20 times the surrounding baseline, slowed their flight, followed circular paths and vocalized frequently, with effects detectable up to 4 kilometers up and even in good visibility.
Then the operators switched the beams off. The behavioral disruption disappeared, and the birds dispersed. Same night, same weather, same place, with only the light changing, which is what turns a repeated event into a rather than an observation.
The scale is what makes it matter: an estimated 1.1 million birds influenced across 7 nights over 7 years, from a single source. The practical conclusion is unusually clean for conservation research, since selectively switching lights off on heavy migration nights is a measure anybody can take tonight. The numbers belong to this installation and not to street lighting in general.
wat. When the beams went dark the concentration of birds dispersed within minutes, which is as direct as evidence of attraction gets.
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Harm can be reduced in real time, because monitoring and a temporary blackout let a flock carry on, which is a far easier ask than dismantling a light.
How do we know?
Van Doren and colleagues monitored the beams with radar and acoustic sensors across seven years, and modeled disorientation and attraction with simulations. They compared the minutes when the lights were on against the minutes when they were off.
What is still unknown
This is one extraordinarily powerful light source, so the numbers do not transfer to a street lamp or to a city in general. The study counts behavior rather than deaths, so the collisions avoided are an inference.
Learn something?
Where to next
Do migrating birds actually see the magnetic field?migration
Which bird travels between both poles every single year?migration
What decides where billions of migrating birds come down to rest?birds